Turbines for generating useful work from water are well known, and many blade types are available. Ideally water leaving a turbine would leave with zero velocity. This would mean that all of the velocity energy entering the turbine would be converted to rotational energy within the turbine for useful work. Pelton wheels are common; however, a great amount of the velocity energy in the water is wasted. The water in a pelton wheel may splatter out of the sides and, upon leaving the pelton wheel, the water still has remaining velocity. Other enclosed turbines are available to restrict water from leaving the sides of the turbine, but such designs still leave water with remaining velocity upon discharge from the turbine.
In addition to bringing the water to zero velocity, an ideal turbine would have blades shaped in such a way so that all of the velocity energy striking the blades would act to rotate the turbine in the same direction. In some turbines with curved blades, the curvature of the blades means that for certain positions of the blades part of the energy from the water will produce a counteracting torque that tends to rotate against the direction the turbine is rotating. Elimination of this counteracting torque would increase efficiency of the turbine.
In addition to poor utilization of available energy, inefficient turbines will require a larger head pressure to produce the same useful work as a more efficient turbine. Since an efficient turbine requires less head pressure, there are more available choices for locating an efficient turbine in places that do not have a large enough head to operate an inefficient turbine.